I could imagine a scenario where gal/ft² is useful. Like with grocery store shelving figuring shelving and product stacking. If liquid storage containers are stackable then you have have more gallons per square footage of shelf space. Or of they're not stackable, then taller containers would hold more liquid in the same shelf space than shorter containers with the same footprint.

Yeah it seems odd to represent something as a volume/area, but that is the relevant information you're comparing and it's intuitive how that number changes based on changes to volume as projected onto an area. Bigger number points toward a more efficient use of shelving.